1. Field of the Invention
The present invention relates to a laser beam irradiation method and a laser irradiation apparatus for using the method (apparatus including a laser and an optical system for guiding laser beam emitted from the laser to an object to be illuminated). In addition, the present invention relates to a method of manufacturing a semiconductor device, which includes a laser beam irradiation step. Note that a semiconductor device described here includes an electro-optical device such as a liquid crystal display device or a light-emitting device, and an electronic device that includes the electro-optical device as a part.
2. Description of the Related Art
In recent years, an extensive study has been made on a technique in which an amorphous semiconductor film formed on an insulating substrate made of glass or the like is crystallized so that a semiconductor film having crystal structure (hereafter referred to as crystalline semiconductor film) is obtained. As the methods of crystallization such as a thermal annealing method using furnace annealing, a rapid thermal annealing method (RTA method), a laser annealing method and the like were examined. Anyone thereof or combining two or more methods thereof can be carried out for crystallization.
In comparison with an amorphous semiconductor, a crystalline semiconductor film has extreme high mobility. Since thus, the crystalline semiconductor film is used to form a thin film transistor (referred to as TFT), for example, the TFT can be widely used in an active matrix liquid crystal display device in which TFTs for pixel portion, TFTs for pixel portion and TFTs for driver circuit are formed on one glass substrate.
Generally, in order to crystallize an amorphous semiconductor in annealing furnace, a thermal treatment at 600° C. or more for 10 hours or more is required. A quartz is an applicable material of substrate for this crystallization, but the quartz substrate is too expensive in price to be manufactured especially in a large area. In order to improve the productivity efficiency, manufacturing the substrate in a large area is unavoidable, it is expected that a substrate in which a length of one side exceeds 1 m will be also used in recent years.
On the other hand, a method of thermal crystallization by using metal elements disclosed in Japanese Patent Application Laid Open No. 7-183540 enable the crystallization temperature which was a conventional problem to be realized at a low temperature. The crystalline semiconductor film can be formed by this method in which a small amount of an element such as nickel, palladium and lead is added to an amorphous semiconductor film, then the amorphous semiconductor film is heated for four hours at 550° C.
Since the laser annealing method can deliver high energy only to the semiconductor film without substantially increasing the temperature in substrate, the laser annealing technology comes under spotlight by its appliance in a glass substrate with a low strain point as a matter of course, and a plastic substrate, etc.
An example of the laser annealing method is a method of forming pulse laser beam from an excimer laser or the like by an optical system such that it becomes a square spot of several cm or a linear shape of 100 mm or more in length on a surface being illuminated, and relatively shifting an irradiation position of the laser beam with respect to the surface being illuminated to conduct annealing. The “linear shape” described here means not a “line” in the strict sense but a rectangle (or a prolate ellipsoid shape) having a high aspect ratio. For example, although, it indicates a shape having an aspect ratio of 2 or more (preferably, 10 to 100), it doesn't make any difference from that a shape at a surface being illuminated is being contained in the laser light having rectangular shape (rectangular shape beam). Note that the linear shape is used to obtain an energy density required for annealing an object sufficiently to be illuminated. Thus, if sufficient annealing is conducted for the object to be illuminated, it may be a rectangular shape and a tabletop shape.
However, a crystalline semiconductor film formed by subjecting an amorphous semiconductor film to laser annealing includes a collection of a plurality of crystal grains, and the position and size of the crystal grains are random. TFTs are formed on a glass substrate by patterning the crystalline semiconductor layer in an island shape for device separation. In this case, the position and size of crystal grains cannot be specified. In comparison with the inner of crystal grains, the interface of crystal grains has an infinite number of a recombination centers or a trapping centers caused by an amorphous structure, a crystal defect, and the like. If the carriers are trapped in trapping centers, potential at a grain boundary will be increased and become barriers to carriers, it is known that current transporting characteristics of carriers will be degraded caused by this. However, it is almost impossible to form a channel formation region by using a single crystal semiconductor film while avoiding the influence of a crystal boundary, although crystal characteristics of semiconductor film of channel formation region have a serious effect on the TFT characteristics.
There is a crystal growth technology that is recently attracting attention. In the technology, when a CW laser is illuminated on a semiconductor film with the CW laser scanning in one direction, crystal grains grow connected in the scanning direction thereof, resulting in forming a single crystal elongated in that direction. It is considered that when this method is applied, a semiconductor film that has no grain boundary at least in a channel direction of a TFT can be formed. However, in this method, since a CW laser having a wavelength in a region that can be sufficiently absorbed by the semiconductor film is used, only a laser that is very small in its output such as substantially 10 W can be applied. Accordingly, in view of productivity, it is inferior to technology that uses an excimer laser.